Tangential cutting inserts, also known as on-edge, or lay down, cutting inserts, are oriented in an insert holder in such a manner that during a cutting operation on a workpiece the cutting forces are directed along a major (thicker) dimension of the cutting insert. An advantage of such an arrangement being that the cutting insert can withstand greater cutting forces than when oriented in such a manner that the cutting forces are directed along a minor (thinner) dimension of the cutting insert.
There is disclosed in EP 0 769 341 a face milling cutter employing a double-sided indexable tangential cutting insert having a prismoidal shape with two opposed generally rectangular rake surfaces connected by side surfaces. The cutting insert has a basic “negative” geometry and therefore in order to provide the necessary clearance between the cutting insert and the workpiece, when mounted in a face-mill, the cutting insert is oriented with a negative axial rake angle. However, negative axial rake angles are disadvantageous, e.g., they have been found to be deficient in cutting efficiency for applications involving difficult to machine materials.
There is disclosed in WO 96/35536 a double-sided indexable tangential cutting insert which when mounted in a face-mill has a positive axial rake angle, even when the necessary clearance between the cutting insert and the workpiece is provided. This cutting inserts presents two peripheral cutting edges for a right-hand face mill and two peripheral cutting edges for a left-hand face mill.
A double-sided indexable tangential cutting insert for a boring tool head is disclosed in U.S. Pat. No. 5,333,972. The insert is provided at each end with a protruding flat island. Each long cutting edge is inclined at an angle of 3° relative to the protruding flat island, defining an “insert axial rake angle”. Rearward of each cutting edge is a descending land surface that merges with an increasing incident angle surface to form a chip breaker groove. Each increasing incident angle surface extends from its associated descending land surface to an adjacent island, at either the top or the bottom of the cutting insert. The cutting insert is left or right handed. It is manufactured to be right-handed and, when flipped around, is left-handed. It will be appreciated that the magnitude of the insert axial rake angle is limited for practical reasons. Any increase in the insert axial rake angle will result in an increase in the “vertical” extent of the increasing incident angle surface (see FIG. 3 of U.S. Pat. No. 5,333,972) that will have an adverse effect on chip development and evacuation.
It is an object of the present invention to provide a double-sided indexable tangential cutting insert that substantially overcomes the above-mentioned problems.
It is a further object of the present invention to provide a double-sided indexable tangential cutting insert having four main cutting edges, for a given direction of rotation of the milling cutter, each main cutting edge having a positive axial rake angle when mounted as an operative cutting edge in a milling cutter.
It is yet a further object of the present invention to provide a double-sided indexable tangential cutting insert capable of milling a square shoulder in a workpiece.